The invention generally relates to thin film deposition and, more particularly, to deposition of metal oxide films.
Thin films are employed at present for a wide range of industrial purposes. For example, thin films find particular utility in the electronics field as a photo-resists during photo-lithography. Such photo-resists are used to define patterns on a substrate for conductive paths, diffusion, ion implantation, etc., to create large-scale intergrated circuits. Thin films, particularly insulating thin films, also find use in passivating electronic circuits after they are formed.
Protective thin films also find applications outside the electronic field. For example, thin films have been used or suggested for use to protect articles, devices or other structures of a variety of materials from attack by environmental agents. Examples of such articles and devices range from works of art to industrial machines.
Numerous methods are known in the prior art for film deposition, including sputtering, evaporation, plating, reactive chemical processes and the like. However, these deposition techniques require considerable expenditures of energy, either in the form of heating of materials, induction of RF fields, plasma formation or laser initiation of chemical reactions.
Sputter deposition involves the ejection of material from a target material source following the impact of energetic sputtering ions. Typically, the target material source and the receiving substrate are arranged within an evacuated chamber in spaced relation on opposing electrodes. A potential impressed across the electrodes serves both to produce a gas plasma between the electrodes and to cause gas ions from the plasma to bombard the raw material source. The ion bombardment knocks off molecules of the raw material, or locally heats the raw material, so as to boil off or evaporate the molecules, causing them to fly in all directions. Some of these projectiles have trajectories which cause them to impinge upon the substrate, eventually building up a thin film. While the film thickness can be fairly accurately controlled, the impacting molecules have high kinetic energy and can damage delicate substrates through the momentum of the collisions themselves or through transformation of the kinetic energy into internal excitation and local heating.
In another method, commonly known as "evaporation" or "thermal vapor deposition", the raw material to be deposited is placed in a heated crucible arranged within an evacuated chamber and heated to a temperature at least sufficient to cause evaporation. The substrate is arranged in a position over the crucible facing the raw material, so as to intercept the evaporant, which is deposited on the facing surface of the substrate. Unfortunately, in some applications, the high temperature of the evaporant can damage heat-sensitive substrates.
In still another method, commonly known as "ion plating" or "plasma plating", conductive raw material is placed within an evacuated chamber opposite a conductive substrate. A high voltage DC field is produced between the raw material and the substrate, the latter being the cathode of a high voltage DC circuit. The chamber is filled with a gas at a pressure sufficient to generate and sustain a plasma discharge. The raw material is then vaporized to form a vapor deposit on the substrate. In the presence of the plasma, a portion of the vaporized raw material becomes ionized, and the positively charged evaporant ions and positively charged gas ions are accelerated by the electric field and bombard the substrate surface to densify the vapor-deposited coating.
A variation of plasma deposition has been developed for depositing thin film, non-conductive or semiconductor coatings on a substrate. In this technique, raw materials are vapor deposited on the substrate, while the substrate is bombarded by ions of a selected gas. During the process, the substrate is maintained in an atmosphere of the selected gas, and an r.f. field is established to produce a plasma of the gas in the vicinity of the substrate. The substrate is electrically biased to attract ions from the plasma which impact on the substrate and densify the vapor deposit thereon. A disadvantage with plasma deposition techniques is that the ions impact the substrate at high kinetic energy causing local heating and possible damage to surface material of fragile or heat sensitive substrates.
Yet another deposition technique developed by one of the present inventors and colleagues is described in U.S. Pat. No. 4,340,617 issued July 20, 1982, in which it is disclosed that thin film depositions can be achieved by photo-decomposition of a gaseous material, such as trimethyl aluminum or dimethyl cadmium. In one embodiment, the gaseous material is photolytically decomposed by a laser source of energy operating at a wavelength less than 700 nm to induce metal deposition on a substrate.
There exists a need for improved deposition techniques which can provide films of selected thickness and density, regardless of the type or conductivity state of the substrate. Such techniques would be preferably gentle to the substrate surface, involving low temperatures, low energy and low impact.
In addition to the gentleness of the deposition process, it is desirable to provide for conformal deposition. The deposited material in such a process should conform to the surface of the substrate, regardless of its form and configuration.
Accordingly, an object of the invention is to provide an improved technique for forming thin films on substrates and, particularly, on heat-sensitive or fragile substrates.
Another object of the invention is to provide an improved technique for protecting exposed surfaces of articles, devices and other substrates against environment attack.
A further object of the invention is to provide thin films useful in the electronics field, for example, as photo-resists or as passivating layers.